1. FIELD OF THE INVENTION
This invention relates to heat exchangers, and more particularly, to techniques for reducing circumferential thermal gradients along the length of the feedwater inlet nozzle.
2. DESCRIPTION OF THE PRIOR ART
Transferring heat from one fluid to another is a common industrial operation. Refineries and chemical processing plants, as well as nuclear and conventionally fueled power plants are typical of the many different installations that make a widespread use of the heat exchanging equipment that is generally required to perform this function.
In a pressurized water nuclear power plant, for instance, a primary coolant fluid extracts heat from the reactor. This hot fluid is circulated to the inlet head of a heat exchanger. In this connection, the associated heat exchanger ordinarily has an inlet and an outlet head, respectively, for receiving and discharging the primary coolant. A bundle or bank of tubes provides primary coolant fluid communication between these two heads. The extreme ends of these tubes, moreover, are customarily anchored in flat tube sheets that serve as closures for the individual heads.
A pressure shell encloses the tube bundle in order to establish a chamber in which a secondary coolant fluid, flowing between the inside surface of the pressure shell and the outer surfaces of the tubes absorbs heat from the primary coolant that is within these tubes. The secondary coolant, usually admitted to this chamber through feedfluid inlets, after absorbing heat from the primary coolant is discharged from the heat exchanger through outlets for distribution to the electrical power generation equipment within the plant.
Because the primary coolant usually is under a pressure that is in excess of 2,000 pounds per square inch, many of the structural portions of the heat exchanger which are subjected to this high pressure necessarily must be formed from thick steel sections. This is especially noticeable in the tube sheets. Each tube sheet, for example, might be pierced by more than 15,000 holes in order to receive and secure the individual tubes in the associated bundle. To provide adequate structural integrity in these circumstances, the tube sheets can be as much as 24 inches thick.
The differences in the primary and secondary coolant temperatures that are experienced within the heat exchanger, however, tend to produce thermal gradients which result in thermal stresses. Thus, for example, the temperature difference that is established between the relatively cold secondary coolant from the feedwater inlet nozzle on one side of a tube sheet, and the higher temperature primary coolant on the other side of the tube sheet, can produce unrelieved forces of great magnitude.
This physical phenomenon, moreover, appears in the feedwater inlet spray nozzles commonly found in heat exchangers of the once-through vapor generating type. In this type of heat exchanger, the inlet feedwater nozzles are disposed within an annular flow path between the vapor generating chamber and the heat exchanger shell. Moreover, "bleed steam" withdrawn from the vapor generating chamber is introduced into the annulus to mix with and heat the feedfluid being discharged from a spray plate disposed in the bottom of the nozzles. Thermal gradients and thermal stresses resulting therefrom are particularly aggravated in the inlet nozzles of this type vapor generator during certain operating conditions which either subject the inlet nozzles to large feedwater temperature changes or low feedwater flow rates. Thus, for example, when the heat exchanger is operating at a low flow condition, the nozzles are only partially filled with cold secondary coolant or feedwater. Moreover, the low flow condition allows the "bleed steam" present in the annulus to enter the inlet feedwater nozzles through the bottom located spray plate and congregate in the upper regions of the nozzle. If the temperature difference between the steam in the upper regions of the nozzle and the feedwater in the lower regions is sufficiently great, excessive circumferential thermal gradients occur along the length of the nozzle which result in potential thermal stress problems and reduced fatique life of the nozzle.
In addition, although the vapor generator has been adequately protected from thermal stresses due to large temperature changes of the inlet feedwater, the feedwater nozzle, and more particularly, the nozzle to flange juncture to the vapor generator shell has not been protected. Because this nozzle juncture has not been thermally protected, a minimum inlet feedwater temperature of about 185.degree. F has been imposed on the heat exchanger system, rather than, the normal expected temperature of about 90.degree. F.
Accordingly, the present inlet feedwater nozzles have placed limitations on the operating conditions of the feedwater system with respect to the low flow rates and the minimum inlet feedwater temperatures. Therefore, there is a need to provide industry with a solution to the problem of heat exchanger nozzle thermal gradients at low flow conditions and at lower inlet feedwater temperatures.